Known is a tandem type color laser printer provided with developing rollers and photosensitive drums for colors of yellow, magenta, cyan, and black. There has been proposed a conventional tandem type color laser printer capable of switching its printing operation mode between monochromatic printing mode in which only a developing roller and a photosensitive drum for the color of black are operated and a color printing mode in which all developing rollers and photosensitive drums are operated.
The printer includes a reversible motor and a pivot gear coupled thereto. When the motor is rotated in the forward direction, the pivot gear is pivotally moved to one direction to be engaged with a developing gear provided at a developing unit for the color of black. This, the printer is switched to the monochromatic mode such that the rotation of the motor is transmitted only to the black developing unit. On the other hand, when the motor is rotated in the reverse direction, the pivot gear is pivotally moved to a direction opposite to the one direction to be engaged with all developing gears for all developing units. Thus, the printer is switched to the color mode such that the rotation of the motor is transmitted to the all developing units.